In petroleum processing, aromatic streams are derived from processes such as naphtha reforming and thermal cracking (pyrolysis). These aromatic streams also contain undesirable hydrocarbon contaminants including mono-olefins, dienes, styrenes and heavy aromatic compounds such as anthracenes.
The aromatic streams are used as feedstocks in various subsequent petrochemical processes. In certain of these processes, such as para-xylene production, e.g., from an aromatic stream containing benzene, toluene and xylene (BTX) or toluene disproportionation, hydrocarbon contaminants cause undesirable side reactions. Therefore the hydrocarbon contaminants must be removed before subsequent processing of the aromatic streams.
Moreover, the shift from high-pressure semiregenerative reformers to low-pressure moving bed reformers results in a substantial increase in contaminants in the reformate derived streams. This in turn results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from the aromatic streams.
Undesirable hydrocarbon contaminants containing olefinic bonds are quantified by the Bromine Index (BI). Undesirable olefins, including both dienes and mono-olefins, have typically been concurrently removed from aromatic streams such as BTX by contacting the aromatic stream with acid-treated clay. Other materials, e.g., zeolites, have also been used for this purpose. Clay is an amorphous naturally-occurring material, while zeolites used for this purpose generally are synthesized and are therefore more expensive. Both clay and zeolites have very limited lifetimes in aromatics treatment services. The length of service correlates with the level of bromine reactive impurities (“BI-reactive” impurities or contaminants) in the feedstream. BI-reactive contaminants rapidly age both clay and zeolites. Indeed, although clay is the less expensive of the two alternatives, large aromatic plants can spend more than a million dollars a year on clay. Furthermore, since zeolites are considerably more expensive than clay, their use in removing hydrocarbon contaminants can only be justified by dramatically improved stability in aromatics treatment so that their cycle length is practical.
U.S. Pat. Nos. 6,368,496 and 6,781,023 teach bromine reactive hydrocarbon contaminants are removed from aromatic streams by first providing an aromatic feedstream having a negligible diene level. The feedstream is contacted with an acid active zeolite catalyst composition under conditions sufficient to remove mono-olefins. The aromatic stream may be pretreated to remove dienes by contacting the stream with clay, hydrogenation or hydrotreating catalyst under conditions sufficient to substantially remove dienes but not monolefins.
Other relevant references include, U.S. Pat. Nos. 6,500,996; and 6,781,023.
Although zeolites have proven equal or superior to clay in many commercial applications, clay has at least one remaining advantage. The clay generally produces lower levels of toluene and benzene byproducts. These byproducts are produced in clay treaters containing xylenes and higher aromatics. They are believed to be produced by transalkylation reactions. The zeolite catalyst is apparently more active than clay for aromatics transalkylation at constant olefin removal levels resulting in higher levels of benzene and toluene impurities in the reactor product. There is a need for methods to improve the selectivity of zeolite catalysts.
Following standard clay start-up procedures, the zeolite catalyst is first dewatered (“dried”) using available unit feedstock at the operating temperature of the parallel reactor that is on-stream, such as to a point where the water level in the effluent is <1000 ppm. Once this point (or some other desired level) is reached, the entire unit feedstock is directed to the reactor with dewatered, fresh catalyst so that the parallel reactor is ready to be brought off line and reloaded with fresh zeolite catalyst (or clay). This results in relatively high selectivity to benzene and toluene impurities.
Prior art methods of reducing such selectivity in zeolites have included reduced catalyst loading, reduced reactor temperature, and/or increased unit feed rate, however these responses are negative from an economic standpoint.
What is needed is a new startup procedure that does not require such tradeoffs.
The present inventor has discovered a new start up procedure that surprisingly improves selectivity of zeolite catalysts and in embodiments makes zeolite catalysts less active than clay for aromatics transalkylation and/or results in less benzene and toluene impurities in the reactor product.